Nameless Sprite Editor (NSE for short) is a good tool for adjusting and making overworlds.
Type: Sprite and Icon Editing
Compatible with: Pokemon Ruby, Sapphire, Emerald, FireRed, LeafGreen
Supported OS: Windows
If you think your overworld is too small or too big and want to resize it to your conditions, use Overworld Changer.
BTX Editor is a tool to edit the BTX (texture) files used in the overworld sprites of NDS Pokemon games.